Bacteria which ferment sugars with the production of acids in particular lactic acid as a major metabolic component have been known for a long time. Such bacteria may be found in milk or milk products, living or decaying plants but also in the intestine of humans and animals. Traditionally, these bacteria have been referred to as “lactic acid bacteria”. Lactic acid bacteria designate a rather heterologous group of Gram positive, non-motile, microaerophilic or anaerobic bacteria which ferment sugar with the production of acids including lactic acid and comprise e.g. the genera Bifidobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc and Pediococcus. 
For centuries lactic acid bacteria have been used in the manufacture of food and feed products including most dairy products, and today lactic acid bacteria are essential in the making of all fermented milk products such as yoghurt, cheese and butter. Furthermore, lactic acid bacteria are widely used in the meat processing industry, wine manufacturing industry, the juice manufacturing industry as well as a number of other industries.
Cultures of lactic acid bacteria also find important uses in the biopreservation of foodstuffs.
The publication of a large number of reports documenting that various lactic bacteria beneficially affect the well-being of humans and/or animals have attracted even further interest to this group of bacteria. In particular, specific strains of Lactobacillus or Bifidobacterium have been found to be able to colonize the intestinal mucosa and to assist in the maintenance of the well-being of the hosts.
EP 0 768 375 describes specific strains of Bifidobacterium ssp, that are capable of being implanted in the intestinal flora and being capable to competitively exclude adhesion of pathogenic bacteria to intestinal cells. These Bifidobacteria are reported to assist in immunomodulation and thus in the maintenance of the individual's health. The immunomodulation effect of Bifidobacteria may even be conferred onto unborn children. WO 01/97822, e.g. describes that intake of Bifidobacterium animalis strain Bb-12® by the mother during her pregnancy reduces the occurrence of atopic diseases in children. Also WO 03/099037 describes that Bifidobacterium animalis strain Bb-12® are able to beneficially modify the immune response. According to Masco et al. (2004), Bifidobacterium animalis strain Bb-12® should correctly be referred to as Bifidobacterium animalis subsp. lactis strain Bb-12®.
Probiotic microorganisms have been defined as “Live microorganisms which when administered in adequate amounts confer a health benefit on the host” (FAO/WHO 2002). During the recent years, documentation on probiotic properties of Bifidobacteria and other lactic bacteria has accumulated. In general, the probiotic activity is associated with specific strains. The previously mentioned Bifidobacterium animalis strain Bb-12® as well as Bifidobacterium lactis strain HN019 have been reported as probiotic (WO 01/97822, WO 03/099037, Zhou et al. (2005), U.S. Pat. No. 6,379,663).
Worldwide there is widespread public concern that the number of antibiotic resistant pathogenic bacteria increases dramatically. All available data indicate that the disturbing increase in antibiotic resistant pathogenic bacteria is caused by an extensive and very liberal use of antibiotics in the general population as well as in animal husbandry.
It is a well established fact that many antibiotic resistant bacteria carry genetic determinants, genes, which confer resistance to one or more antibiotics. It is furthermore wellknown that such genetic determinants under certain circumstances are transferable and may confer the antibiotic-resistant phenotype to recipient bacteria. The frequency of transfer is very much dependant on the particular genetic context in which the antibiotic resistance genes are found. I.e. antibiotic-resistant genes residing on plasmids or on transposons have been demonstrated to confer the antibiotic-resistant phenotype to recipient bacteria at relatively high frequencies, whereas chromosomally encoded determinants are very much less prone to move.
For these reasons it may be of concern to ingest even beneficial, non-patogenic bacteria if they do contain an antibiotic resistant determinant. This concern is further emphasized in the report from the EUROPEAN COMMISSION's Scientific Committee on Animal Nutrition (SCAN) on the Criteria for Assessing the Safety of Micro-Organisms Resistant to Antibiotics of Human Clinical and Veterinary of 3 Jul. 2001, revised on 24 Jan. 2003, stating that the presence of a known resistance gene is not acceptable (page 21).
Resistance to tetracycline is the most common bacterial antibiotic resistance found in nature and similarly is the most widely distributed type of resistance among bacteria isolated from animals (Billington 2002). Tetracycline inhibits protein synthesis by binding to a single high-affinity site on the 30S ribosomal subunit. With tetracycline in this position, the binding of aminoacyl-tRNA to the A site is prevented and thus protein synthesis is blocked.
Resistance to tetracycline may be mediated either by active efflux of tetracycline from the cell, by ribosomal protection by one or more soluble protein(s), the so-called ribosomal protection proteins (RPPs), or by enzymatic inactivation of tetracycline.
Recently, a new ribosome-protection-type tetracycline resistance (Tetr) gene, tetW, was identified in rumen isolates of Butyrivibrio fibrisolvens and a number of other rumen bacteria (Barbosa, 1999).
Although the tetW determinant is widely distributed among tetracycline resistant isolates of animal pathogens (Billington 2002), it was a surprise for the authors of this application to find that all known probiotic strains of Bifidobacterium animalis subs. lactis, including the two well-known Bifidobacterium strains Bb-12® and DR10™, carry a functional tetW determinant and are resistant to tetracycline; in particular because in a recent report the DR10™ strain as well as the Bb-12® strain were reported to be tetracycline sensitive (Zhou et al. 2005).
Even though extensive experiments have indicated that the tetW determinant of Bifidobacterium animalis subspecies lactis strain Bb-12® is not movable under realistic situations, the concern of antibiotic resistant determinants in food products still remains. Consequently, we have attempted several approaches to accomplish inactivation or removal of the tetW gene in Bifidobacterium animalis subspecies lactis Bb-12® by classical mutagenesis, involving various mutagens, as well as by direct genetic manipulation. However, until now all attempts have been unsuccessful. It is contemplated that an important reason to the many unsuccessful attempts is the fact that the tetW is located on the chromosome of probiotic Bifidobacterium animalis subs. lactis strains.
Thus it would be highly advantageous to establish a method for the inactivation of the tetW resistance gene in probiotic Bifidobacteriacea. Such method could furthermore help to solve the problem of providing antibiotic sensitive variants of commercial interesting probiotic, tetracycline resistant Bifidobacteriacea, that may meet the requirement of absence of functional antibiotic resistance genes.